Abstract Septic shock is the most frequent cause of death in the intensive care units. The microvascular hyper- permeability resulting in organ tissue edema and the severe hypotension resulting in organ tissue hypo- perfusion are the most detrimental complications of septic patients leading to such a high mortality rate. To date, there are no specific therapies for sepsis. Norepinephrine is used as a first line vasopressor to attenuate hypotension; however, development of host hypo-responsiveness to norepinephrine often requires its use in higher doses, that bring with them unwanted side effects that further increase mortality of septic patients. Large amount of fluids is administered to expand plasma volume and improve tissue perfusion, but, in presences of increased microvascular hyper-permeability, this approach often augments tissue edema and further worsens the tissue hypoxia and injury. Thus, development of safe and efficient therapy that reduces the requirement for both fluid and vasopressors to maintain blood pressure and adequate tissue perfusion is in urgent need. The goal of this translational proposal is to characterize novel mechanistic aspects of microvascular hyper- permeability during methicillin-resistant Staphylococcus aureus (MRSA) sepsis, with special focus on interplay between arginine vasopressin (AVP) V2 receptor (V2R) and potent permeability factors, such as excessive nitric oxide and its metabolites, angiopoieitin-2, and bradykinin and offer efficient therapeutic option that reduces the need for fluid and norepinephrine, and improves survival. Our general hypothesis is that activation of AVP V2R plays a critical role in microvascular hyper-permeability during sepsis and the use of its antagonist tolvaptan, as an adjunct therapy to norepinephrine will alleviate severity of sepsis-induced multi-organ dysfunctions. We further hypothesize that tolvaptan will effectively reduce fluid and vasopressor requirements. To achieve our goal, we propose to study following two specific aims, using clinically relevant ovine model of MRSA sepsis as well as in vitro studies with cultured human pulmonary microvascular endothelial cells. We are particularly optimistic about the possibility that the proposed research will lead to translational clinical studies for AVP V2R antagonists, as an adjunct therapy to norepinephrine, for management of septic shock. Specific Aim 1. To determine that activation of AVP V2R promotes severity of microvascular hyper- permeability and pulmonary edema during MRSA sepsis and septic shock. The goal of this aim will be achieved performing both in vivo ovine and in vitro cultured cell studies. Specific Aim 2. To demonstrate safety and efficacy AVP V2R antagonist, as a potential adjunct therapy, for treatment of septic shock. The goal of this aim will be achieved comparing effects of AVP V2R antagonist tolvaptan with or without norepinephrine in ovine model of MRSA septic shock with special emphasis on cardiovascular collapse.